The present invention relates to color TVs and displaying instruments using CRT(Cathode Ray Tube) and, more particularly, to a circuit which can prevent the degradation of a screen by minimizing the doming caused by excess beam current at the time of initial operation, where "doming" represents the dome-shaped expansion of a shadow mask by heat.
Generally, since the shadow mask of a CRT is thermally expanded by beam current, it must be well designed to be uniformly expanded from a center to an edge of a screen, in order to minimize the degradation of the screen. But, this is possible only when video signal level is the same over the full screen and the thermal expansion occurs slowly.
In practice, the general video signal level is not identical over the full screen and the average picture level over the full screen is different for every picture. Thus, if the video signal has a continuously high average picture level or locally high average picture level from initial operation after power turns on, the shadow mask of the CRT is non-uniformly expanded, yielding the doming phenomenon. Then, misslanding also occurs so that R(red), G(green), and B(blue) beams from an electron gun are not projected accurately onto R.G and B points, placing a stain on the screen and leading to the disagreement of colors on the screen.
Thus, the picture is seriously damaged. As dot pitch, that is, the distance between the color point and the next same color point is smaller, it becomes more difficult to achieve higher resolution and larger size displays.